1. Technical Field
The present invention relates to a sheet conveying device provided in a sheet conveying system, an image reading device including the sheet conveying system, and an image forming apparatus including the sheet conveying system or the like. In particular, the present invention relates to a skew correcting device employed in one of the sheet conveying system, the image reading device, and the image forming apparatus or the like suitable for correcting skew of a thin sheet.
2. Related Art
In various types of image forming apparatuses, such as copiers, printers, facsimiles etc., and an image reading device, such as a scanner, etc., an image formation unit and/or an image reading unit are installed in the apparatus and device. In such an apparatus and device, by conveying a sheet, such as a plain paper sheet, etc., through the image formation unit, a desired image is formed on the sheet. Similarly, by conveying a sheet as an original document with recorded image information thereon through the image reading unit in such a device, the image information on the sheet is optically read.
In the conventional image forming apparatus and image reading device, to properly form an image or read the image information, skew of the sheet is corrected just before respective positions of the image reading unit and the image formation unit in a sheet conveyance direction to adjust a posture and a position of the sheet.
As a sheet conveying device that corrects the skew of the sheet, a roller nip collision system is known, in which a leading end of a sheet collides with a pair of nip forming rollers that stop rotation to bend the sheet and cause the leading end to fit along a roller nip formed between the pair of nip forming rollers by its elasticity.
Otherwise, a gate collision system is provided to correct the skew of the sheet. Specifically, a gate unit capable of entering or evacuating from a sheet conveyance path is provided to stop the sheet and cause the leading end thereof to fit along a gate member. The gate member evacuates from the sheet conveyance path after fitting of the leading end and thereby correcting the skew of the sheet.
However, in the roller nip collision system, such sheet skew correction performance depends on the bending of the sheet. In addition, the number of rollers installed and the spacing of the rollers to form the bending the sheet are limited by the maximum sheet width that the image forming apparatus can handle.
By contrast, in the gate collision system, when the leading end of the sheet collides with a sheet striking member (e.g. the gate), since it is sandwiched, and accordingly restricted at the nip formed between the pair of opposed conveyance rollers, the sheet rarely rotates on a horizontal plane to correct its skew. Accordingly, a degree of freedom to correct a position of the leading end of the sheet is relatively low, and accordingly, a small amount of the skew of the sheet can be corrected. As a result, an image is either diagonally formed or read resulting in inaccurate recording or reading.
Then, in the past, various sheet skew correction technologies are proposed as discussed, for example, in Japanese Patent Application Publication Nos. JP-2002-265100-A, JP-2002-128326-A, and JP-H08-081089-A or the like.
For example, JP-2002-265100-A discloses a technology, in which a pair of roller pieces is disposed at a widthwise center of a sheet to bend the sheet while reducing restriction force applied to the sheet and improving a skew correction function during a skew correction process, thereby upgrading recording accuracy.
Further, JP-2002-128326-A and JP-H08-081089-A also disclose technologies, in which a spherical roller is employed to bend the sheet while further reducing the restriction force applied to the sheet, thereby preventing stains and wrinkles possibly generated in the sheet.
However, with such conventional skew correction technologies, an especially thin sheet having a thickness of about 55 gsm (gram per square meter: g/m2) wrinkles at its leading end during a skew correction process.
That is, with the gate collision system of JP-2002-128326-A or the like as shown in FIG. 10A in which a reference sign H indicates a width of a sheet 4, since a relation between a widthwise interval L between multiple gate members 1 and 1 disposed in an axial direction and a width W of a pair of roller pieces 3 of a conveyor roller 2 in the same direction is represented by an inequality (L<W), a pushing force is generated to push the leading end of the sheet 4 outside the gate members 1 so that sheet buckling 5 and wrinkles 6 are generated sometimes as shown in FIGS. 10B and 10C, respectively. The above-described sheet buckling 5 and wrinkles 6 are likely generated especially when a thin sheet having a thickness of less than about 55 gsm or the like is used.
Similarly, with the system of JP-H08-081089-A shown in FIG. 11, the skew is corrected by bringing a leading end of the sheet 4 to a nip 7 formed between a pair of sheet conveying rollers 2 and rotating the sheet 4 in a prescribed direction 8 regarding the nip as a rotation axis to collide with the nip 7. Therefore, distortion and/or wrinkles 6 are generated at the leading end of the sheet 4.